Nobody can say exactly where a pond ends and a lake begins
Earth may hold around 304 million lakes and ponds, yet there is no internationally agreed rule for telling the two apart. Nine in ten of those water bodies cover a hectare or less, while almost all lake water sits in fewer than a hundred giants, because bigger lakes also tend to be far deeper.
Definitions vary by discipline. Some limnologists treat a lake as simply a large pond, one big enough for waves on the shore or for wind to stir its water column. A hydrology textbook offered five tests instead: the water fills one or more connected basins, keeps roughly the same level throughout, receives no regular seawater, traps much of its suspended sediment, and exceeds some chosen area, such as a hectare. Other hydrologists have accepted most of those points except the one about seawater.
The English word travelled a long road, from Old English lacu, meaning pond or stream, back to an ancient root meaning to leak or drain. Most lakes are freshwater and cluster at high northern latitudes. Canada is thought to hold at least 2 million, of which 31,752 exceed 3 square kilometres. Finland counts 168,000 of at least 500 square metres. Beyond Earth, Saturn's moon Titan has lakes shaped much like ours, and Mars once had them too, though only dry beds remain.
In 1957 the ecologist G. Evelyn Hutchinson published A Treatise on Limnology, which sorted lakes by origin into 11 major types and 76 subtypes, a scheme still widely used. Tectonic lakes fill basins created by movements of the crust; the great rift lakes of Central Africa and Lake Baikal belong here. Glaciers and ice sheets gouge or dam many more. Crater Lake in Oregon fills the caldera left when Mount Mazama collapsed around 4860 BC, and volcanic crater lakes come closer to a perfect circle than any other type.
Rivers make lakes as well. A meandering river erodes the outside of each bend faster than the inside until a loop becomes a horseshoe; eventually the water cuts across the narrow neck and abandons the curve, leaving a crescent called an oxbow lake. Some lakes have no outlet at all and lose water only through evaporation or seepage. In dry rift basins, these endorheic lakes often turn salty.
Source: Lake
